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Hi, Asianbooblover; you started out by asking about direction of sex scenes, and then moved on to scripted scene direction, as with the "r***ing in Front of Husband" genre (if we may call that a genre).
If it's sex scene direction, that would depend on the director and style of movie. I'm sure there are times when the director would allow free rein ("Have fun, kids! Do whatever you'd like"), but most of the time, your "outline" suspicion would need to come into play. While the regimen can't be strict, I'm sure a director would require a sex scene "build up" and the actors would be instructed on what they would need to do. First, for example, lady gives a blowjob. Next, missionary. Then, doggie, then, cowgirl, etcetera... finished by the requisite "Put-dick-in-mouth" after the cumshot. If one has seen enough JAV, even the sex scenes can sometimes appear to be follow-the-numbers, with few surprises.
A movie is shoot-and-stop, shoot-and-stop (instead of one continuous take, as with Alfred Hitchcock's ROPE; although we are also familiar with the "hidden camera" type of JAV where one uninterrupted angle can go on for a long time... yet even in that type of movie, there are always breaks), so there is much room to maneuver; if a sex scene is hot, no doubt the director would suggest, after the next momentary break, Let's try the same sexual position from this camera angle!
As far as the scripted scene direction, as with "r***ing in Front of Husband," no doubt it would be more regimented. There is a script to follow, and these pornos are shot as any regular type of movie. Now the question is, because porn actors are not always real actors, how much are they required to stick with the written lines? We know as English-speakers, let's say with American porn, that even with "dramatic story" porn, there is a lot of leeway. More often than not, the actors are told of the nature of the scene, and we can tell what is performed has been improvised. This is to encourage that genuine feel; when non-actors are asked to stick to scripted lines, there's always a danger of the scene coming across as stilted... as we have seen often in 1970s American "real movie" porn. (Most porn from those days were "real movies," in other words, with hardcore sex in between.) That period, in fact, is why porn actors have gotten their reputation as being laughable actors. (Obviously; there is a higher consideration that takes place with porn casting, taking priority over acting ability.)
This procedure describing U.S. story porn is undoubtedly the way "story" JAVs are shot, even if we don't understand the language. (Although on the surface, I'm often impressed with the way JAV actresses handle their lines. In contrast, when I watch Western porn, I usually turn the audio off. One could almost always count on the American actresses ad-libbing with remarks that don't follow their character, ruining the illusion of the set-up.) I'm sure there are cases where directors can rely on especially talented JAV actresses to recite their lines, as written. (As any other "real" actress would.)
(Actually, I usually watch JAV with the audio off, as well. I can't stand listening to those little-girl squeals.)
The other type of JAV is gonzo porn, as with, for example, the "Nampa" movies. As with their U.S. counterpart (let's say, "Bangbus"), the acting is a lot more free-for-all. Yet while improvisation is the rule for these types of movies, there is always an outline to follow.
(Disclaimer: I'm not a porn-maker. What I've described just seems like common sense.)
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